Acute inflammatory reactions have been produced in hamster lungs by the intrapulmonary instillation of preformed chemotactic mediators (C5a and F-Met-Leu-Phe). By the use of 111 Indium and 125 Iodine labeling of homologous neutrophils (PMNs) and homologous albumin, respectively, it has been possible to obtain highly reproducible and quantitative parameters of the acute inflammatory response. The lung responds in a dose-dependent and time-dependent manner to the instillation of preformed chemotactic mediators. As expected, the quantitative parameters indicative of PMN influx were more prominent than changes in vascular permeability. The data obtained by the use of radiolabeled PMNs were confirmed by observation with light microscopy. Non-chemotactic substances such as human IgG, serum albumin and C4 failed to induce inflammatory responses in lung. Interestingly, intact C5 instilled into lung was extremely phlogistic, apparently due to hydrolysis once within the lung. These studies provide an approach to reliable quantitative parameters of inflammatory reactions in the lung and emphasize the in vivo biological effects of chemotactic mediators.